The long term goals of my research are to understand the molecular mechanisms of specific cell-cell recognition and establish how these events at the cell surface lead to secondary responses by cells. I have chosen the mating reaction (fertilization) of the biflagellated alga, Chlamydomonas reinhardtii, as a model system for these studies. Fertilization is initiated in this organism when gametes of opposite mating types (mt+ and mt-) adhere to each other via adhesion molecules, agglutinins, on their flagellar surfaces. Agglutinin interactions initiate a complex series of events, termed gamete activation, that prepare the interacting gametes for cell fusion. Early during gamete activation, flagellar protein kinases couple agglutinin interactions to activation of adenylyl cyclase. Later, agglutinins synthesis is upregulated and agglutinins translocate to the flagella. We have identified three biochemical events that are markers of gamete activation, including inhibition of a flagellar protein kinase; phosphorylation of a newly discovered mt+ gamete specific homeodomain protein, GSP1; and gamete activation-induced translocation of a recently discovered aurora/IpI1-like protein kinase from the cell body to the flagella. The research described in this application aims to accomplish the following: 1) Study the biochemical and molecular mechanisms that couple increases in cAMP to phosphorylation of GSP1 homeodomain protein; 2) Examine the function of GSP1 in mt+ gametes; 3) Investigate cellular and molecular properties of the CALK aurora/IpI1-like protein kinase; and 4) Study regulation of the phosphorylation state of the protein kinase, SksC. These studies should provide new information about biochemical and molecular mechanisms of gamete activation and cellular responses induced by cell-cell interactions, which are important in the proper functioning of the immune system, in metastasis, and in fertilization and early development.